A multi-antenna multiple-input multiple-output (MIMO) technology has been widely applied in a wireless mobile communications system to improve spectrum efficiency and cell coverage. For example, a Long Term Evolution (LTE) downlink supports transmitting on 2, 4, and 8 antenna ports. To better support downlink transmission on multiple antenna ports, an evolved NodeB (eNB) generally uses a precoding matrix to perform preprocessing or precoding on data that needs to be transmitted, so as to reduce interference between different data streams in single user MIMO (SU-MIMO), or data stream interference between different users in multiple user MIMO (MU-MIMO), and increase a signal to interference plus noise ratio (Signal to Interference and Noise Ratio, SINR) of data.
Information required by preprocessing is based on downlink channel measurement information that is fed back by user equipment (UE). The UE performs channel estimation according to a reference signal transmitted by the eNB, for example, a channel state information reference signal (CSI-RS), and determines CSI according to an estimation result. The CSI includes information such as a transmission rank (that is, a quantity of layers of transmitted data), a precoding matrix, and a channel quality indicator (CQI). Then the UE feeds back the determined CSI to the eNB. The CSI fed back by the UE provides merely a reference for the eNB performing downlink scheduling. Specifically, how to use the CSI fed back by the UE is determined by the eNB.
Generally, for each rank, a specific quantity of precoding matrices is designed for representing quantized channels. The designed precoding matrices form a codebook. Each precoding matrix in the codebook has an identifier, that is, a precoding matrix indicator (PMI). The codebook is predefined, that is, both the eNB end and the UE end store a same codebook, and a correspondence between each precoding matrix and each PMI in the codebook is understood consistently. After the UE selects a precoding matrix from the defined codebook according to the estimated downlink channel, the UE needs only to feed back a PMI corresponding to the selected precoding matrix to the eNB, and the eNB may determine the specific precoding matrix according to the PMI fed back by the UE.
Because a precoding matrix represents channel state information, a codebook design affects system performance. The codebook design is directly related to a specific configuration of a transmit antenna on the eNB side. Using 2, 4, and 8 antenna ports supported in LTE Rel-8/9/10 as an example, during configuration, it is assumed that all the antenna ports are arranged in a same dimension, that is, in a horizontal direction, as shown in FIG. 1A and FIG. 1B. FIG. 1A shows a schematic diagram of a uniform linear array (ULA) arrangement manner of 2 antenna ports and 4 antenna ports. FIG. 1B shows a schematic diagram of a cross polarization arrangement manner of 2 antenna ports, 4 antenna ports, and 8 antenna ports.
Because all the antenna ports are arranged in the horizontal direction, a function of the precoding matrix is to adjust a phase on each antenna port to generate a horizontal beam pointing to the UE. As shown in FIG. 2, 4 beams PMI 1, PMI 2, PMI 3, and PMI 4 serve UE 1, UE 2, UE 3, and UE 4 respectively, so as to increase an SINR of a signal. Such an antenna configuration is suitable for serving users in a cell that are all distributed on a same horizontal plane (for example, all users are distributed on the ground). By using multiple different precoding matrices, the eNB can simultaneously generate multiple beams in different directions to serve different UEs in a MU-MIMO manner.
However, in a practical environment, UEs in a cell are generally distributed in two dimensions: in a horizontal direction and in a vertical direction. For example, UEs in a cell are distributed in different buildings and on different floors of a same building. Because the current antenna port configuration can control a beam direction only in the horizontal direction, UEs at different heights in the vertical direction cannot be better served. Currently, due to development of an AAS technology, transmit antennas or antenna ports on the eNB side can be easily arranged in the two dimensions: in the horizontal direction and in the vertical direction. Antenna ports in the same horizontal direction can control the horizontal direction of the beam as before (for example, antenna ports 0, 1, 4, and 5, or antenna ports 2, 3, 6, and 7 in FIG. 3). At the same time, antenna ports in different rows can control the vertical direction of the beam (for example, the vertical direction of the beam may be controlled by combining the antenna ports (0, 1, 4, and 5) and the antenna ports (2, 3, 6, and 7) in FIG. 4). The two-dimensional antenna port configuration can freely control the horizontal and vertical directions of the beam to serve more UEs, as shown in FIG. 4.
However, in the current LTE system, all precoding matrices are designed for antenna ports that are all arranged in the horizontal direction, and can generate only the horizontal beams. When the antenna ports are distributed in a two-dimensional manner, the existing precoding matrices cannot control beam directions in horizontal and vertical directions simultaneously. Therefore, the existing precoding matrices cannot control beams in horizontal and vertical directions.